Bob McDavitt's ideas for sailing weather around the South pacific

01 August 2009

BOBGRAM7 issued 2 August 2009

Issued 2 August 2009
Bob McDavitt's ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas come from
the patterned world of weather maps, so please fine-tune to your place.
Dates are in UTC unless otherwise stated.

The South Pacific Convergence Zone reformed last week at the northern
extent of its range - across Solomons/Tuvalu to Samoa, and then east
southeastwards across French Polynesia FP .

This week the SPCZ is likely to drift southwards as a new Madden Julian
Oscillation (boost of tropical convection) advances into the western
Pacific from Australia. By the end of this week it should stretch
across the Coral Sea towards the Loyalty Islands area/maybe New
Caledonia and then southeastwards towards Kermadecs. Another branch is
likely to stretch from Northern Tonga to southern parts of French

A tale of two lows; At the south end of each branch of the SPCZ a low
is likely to form this week, disrupting the otherwise steady trade wind
flow. The Low that is forecast to form near 20S 140W on Thursday 6 Aug
UTC (call it L1) should move southeast and deepen below 10000, and is
likely to kill the winds over the main part of FP in the process.

The low that is likely to form near New Caledonia or the Loyalty Islands
on or around Sat 8 Aug UTC, call it L2, is at this stage forecast to
deepen to around 1006 and become complex as it moves south towards North
island early next week.

Avoid these lows, or maybe use them as clockwise wind swings.

During the coming week a weak trough is likely to remain
quasi-stationary to north of 30S between NZ and Fiji: Its eastern side
is wet with northerly winds, and its western side is relatively dry with
S to SE winds.

The remains of last weeks big fat high BFH is quasi-stationary near 35S
115 to 145W, enhancing the trade winds between 20 and 25S on its
northern side. However, its western flank is expected to fade when L1
forms over southern FP around 6 Aug. Avoid the squash zone between the
high and L1.

A weak high cell is expected to chug along 32-35S from east of NZ on Mon
3 Aug to 170W on Thu 6 Aug and there fade.

A new high cell is likely to form in the Tasman Sea on Thu 6 Aug and to
cross central NZ on Fri 7 Aug and then build to over 1030 east of NZ,
near 35-40S 160-170W, this weekend and early next week. Beware the
squash zone between it and L2--- these strong NE winds make sailing from
NZ to Tonga a bad idea from 8 to 12 Aug.

A long-wave trough LWT is stuck over the Southern Bight and its
ridge-buddy is stuck over 180. The LWT is generating fronts and tossing
them across the Tasman at the rate of around one every other day.

These fronts do not really have attached lows associated with them and
vary in intensity and are generally decaying as they cross NZ, but have
guts in them when the reach our SW end. There is one crossing the
North Island this evening (Sunday 2 Aug), next one is due Tuesday and
then a weak one on Thursday and another on Saturday. These fronts are
preceded by NW flows that concentrate the rain on the western slopes of
the main divide and bring Foehn warming downwind into the rain-shadow.
Yes, this is typical of an El Nino.

The terms used are more fully explained in the METSERVICE Yacht Pack.
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